1. Field of the Invention
The invention relates to a liquid crystal compound having a cyclohexene oxide moiety as a polymerizable group, its polymer and a film having optical anisotropy.
2. Related Art
A polymerizable liquid crystal compound has recently been utilized for an optically anisotropic substance such as a polarizing plate and an optical retardation plate. This is because the compound exhibits optical anisotropy in the liquid crystal state and then the liquid crystal molecules in this state are fixed by polymerization. The kinds of optical characteristics required for a polymer having optical anisotropy depends on a purpose for use, and a compound having characteristics that suits the purpose is necessary. In the compound used for such a purpose, characteristics regarding its polymer are important in addition to the characteristic of optical anisotropy. Characteristics required for the polymerizable liquid crystal compound, its composition and their polymer include compatibility, solubility, clearing points, applicability, the rate of polymerization, transparency, melting points, the degree of crystallinity, glass transition temperature, shrinkage, water permeability, water absorptivity, mechanical strength, chemical resistance and thermal resistance.
A compound having an acryloyloxy group as a polymerizable group among polymerizable liquid crystal compounds is used for such purposes (patent documents Nos. 1 and 2). The acrylate has a high reactivity and the polymer derived from it has a high transparency. However, it is necessary to carry out the reaction in an atmosphere of an inert gas and to increase energy of ultraviolet irradiation, because the mode of polymerization is radical polymerization. Thus, an improvement of workability on curing in air and also of characteristics such as heat resistance, shrinkage, adhesive properties, adhesion and mechanical strength is required. Furthermore, a polymerizable liquid crystal compound (or its composition) is diluted with an organic solvent for the purpose of adjusting applicability and can be used as ink. The ink in which the viscosity, the leveling properties and so forth are adjusted is prepared by dissolution of the polymerizable liquid crystal compound (or its composition), a photopolymerization initiator, a surfactant and so forth in an organic solvent in the case where a film having optical anisotropy is produced from the polymerizable liquid crystal compound (or its composition). The ink is applied to a transparent substrate film that is aligned, the solvent is dried, and then the polymerizable liquid crystal compound (or its composition) is oriented. Next, the compound is polymerized with ultraviolet irradiation or heat and thus the oriented state is fixed. An organic solvent such as propylene glycol monoethyl ether acetate (PGMEA) is desirable for a solvent used in this procedure in view of environmental load and safety (mutagenicity and toxicity). However, the compounds disclosed in non-patent documents Nos. 1 and 2 have not a large solubility in such a solvent for preparing the ink with a suitable concentration. Incidentally, although compounds similar to the compound of the invention are disclosed in claims of the patent documents Nos. 3 to 6, there are no specific descriptions of the compound of the invention and its effect in any of these documents.
Conventional compounds are disclosed in the following patent documents: No. 1, JP H07-017910 A (1995); No. 2, JP H09-316032 A (1997); No. 3, JP 2008-239567 A; No. 4, JP 2008-110948 A; No. 5, JP 2008-239569 A; and No. 6, JP 2010-083781 A.
Conventional compounds are also disclosed in the following non-patent documents: No. 1, Polymer Chemistry, 1993, 31(9), 2249-60 and No. 2, Polymer, 1994, 35(3), 622-9.